Electroplating of zinc-tin alloys



E. B. SAUBESTRE ET AL 2,898,274

ELECTROPLATING 0F ZINC-TIN ALLOYS -1O Sheets-Sheet 1 Aug. 4, 1959 Filed Aug. 8, 1955 FIG. I.

ZINC IN ALLOY (Z) 'INVEN TOR5 EDWARD B. $AUBE$TRE ARNOLD D.ARNAUT Wan/11M ATTORNEY 1959 E. B. SAUBESTRE ET AL 2,898,274

ELECTROPLATING OF ZINC-TIN ALLOYS Filed Aug. 8, 1955 10 Sheets-Sheet 2 FIG. 2.

CURRE NT EFFICIE NCY INVENTOR5 EDWARD B. SAUBESTRE.

ARNOLD D. A NAUT BY Nam ATTORNEY g 1959} E. B. SAUBESTRE ETAL 2,898,274

' ELECTROPLATING OF ZINC-TIN ALLOYS Filed Aug. 8, 1955 l0 Sheets-Sheet 3 F|G.3. I

zmc IN ALLOY INVENTORS EDWARD 5. SAUBESTRE.

ARNOLD 0. A NAUT NM ATTQRNEY Aug. 4, 1959 l0 Sheets-Sheet 4 Filed Aug. 8, 1955 FIG. 4.

CURRENT EFFICIENCY INVENTORS EDWARD B. SAUBESTRE ARNOLD D- AQNAUT M; flwil ATTORNEY Aug. 4, 1959 E. B. SAUBESTRE ET AL 2,898,274

ELECTROPLATING 0F ZINC-TIN ALLOYS Filed Aug. 8, 1955 10 Sheets-Sheet 5 FIG. 5'

zmc m ALLOY(%) IN V EN TORS EDWARD 5. SAUBESTRE. ARNOLD D. ARNAuT Wm M 2 ATT hum Aug. 4, 1959 E. B. SAUBESTRE ET AL 2,398,274

ELECTROPLATING OF ZINC-TIN ALLOYS Filed Aug. 8, 1955 10 Sheets-Sheet 6 CURRENT EFFICIENCY IN V EN TORS EDWARD a. SAUQLSTRE ARNOLD D. ARNAUT Ma-M ATTORNEY E. B. SAUBESTRE ETAL 2,898,274

ELECTROPLATING OF ZINC-TIN ALLOYS Aug. 4, 1959 10 Sheets-Sheet 7 Filed Aug. 8. 1955 zmc IN ALLOY(%) INVENTORS EDWARD a. siauaasma ARNOLD 0. ARNAUT BY m v A-r'roaNiY Aug. 4, 1959 E. B. SAUBESTRE ETAL 2,898,274

ELECTROPLATING OF ZINC-TIN ALLOYS 10 Sheets-Sheet 8 Filed Aug. 8, 1955 FIG. 8

CURRENT EFFICIENCY Aug. 4, 1959 E. B. SAUBESTRE ETAL ELECTROPLATING OF ZINC-TIN ALLOYS 10 Sheets-Sheet 9 Filed Aug. 8, 1955 FIG. 9.

INVENTORS EDWARD a. SAUBESTRE A'r'roRNEY Aug. 1959 E. B. SAUBESTRE ETAL 2,898,274

ELECTROPLATING OF ZINC-TIN ALLOYS Filed Aug. 8, 1955 10 Sheets-Sheet 10 FIG. IO.

CURRENT EFFICIENCY IN VEN TORS EDWARD B. $AUBE$TRE ARNOLD D. RNAUT TTORNiY United States Patent 'fiice 2,898,274 Patented Aug. 4, 1959 ELECTROPEATING OF ZINC-TIN ALLOYS Edward B. Saubestre, Eimhurst, and Arnold D. Arnaut,

Syosset, N.Y., assignors, bynmesne assignments, to Sylvania Electric Products. Inc., a corporation of Dela.- ware Application August 8,1955, Serial No. 526,860

22 Claims. (Cl; 204-43) larly in the electronics industry, there has been a need: for a substitute for: cadmium which of recent; times has.

become both scarce and expensive; such replacement material ideally should have the corrosion resistant propertiesof cadmium at a lower cost, coupled'withthe ready solderability. of cadmium. In anattempt to meet such need, it has been suggested in the prior art to provide a tin-zinc alloy containing at least 50% tin and usually of the order of 70m 85% tin, balance zinc. Such a high tin alloy provides definite advantages as compared to zinc, prominently in that it isreadily soldered with tin lead:

solder using non-corrosive conventional solder. fluxes.

However, these high tin alloys, of the. order of 75% tinconcentration, in general do not give the. basis metal. as

good corrosion protectionas does the purezzinccoating;

and the relatively high concentration of'tin serves to raise the plating metal cost to' a value many times thatzofpure zinc.

' Accordingly, it'is broadly an objecttof thepresent-inventionto provide improved plating methods-andbaths-ob- Vlatlng.;()n61OI more ofxth'e: aforesaiddifliculties;v Specifically, it :is within the. contemplation of thepresent ,inven-r tionto provide an.electrodepositedtalloy of zincrandtin having a quantityoftin .sufficient :torender the :alloy. sol

derable, while notappreciably.alteringithe otherchemical. To advantage, solderable zinc. alloys properties of zinc. deposited. according to' the present;inventionghave the corrosion resistant properties,comparablesto that of pure;

zinc. at platingmetalzcosts muchalowerthan .that-of'conventional tin-zinc alloys or cadmium.

solutions.

the, metal is-present as the: anion.

Sn(OI-I) It has been found in conventionaltin-zinc alloy baths that increasingthe concentration of zinc salts:

in'thebath increases the :ease of deposition of zinc. This may be attributed to arr-increase inv the zinc ion concentration for a,- fixed concentrationof cyanide and hydroxide ions. have no effect, per set on the ease .ofdeposition of tin.

However, regarding the effect ofihydroxideqandcyanide on. the ease of deposition-of'zinc, the situation. is complicated by the-fact that-zinc is complexedrwith both hydroxide and cyanide. the.;-plat ing of zinc,occurs ,almost entirely fromythehy droxide complex. Angincrease in hydroxide increases the,

content of the complex from which zinc plating occurs.

Further, the cyanide content hasbeen found to- It is now generally believed that:

- Such known plating baths as: described in the literature and in commercial operating manuals on tin-zinc plating may be characterized aspositive: free cyanide baths. Free: cyanide is conventionally'defined as the difference between the amount offcyanide theoretically required to. complex allof the zinc as cyanozincate and the total amount of cyanide present. Each mole of zinc present requires four moles of sodium or potassiumcyanide to be complexed'or-inweight units:

Free sodium cyanide=Total sodiumcyanide3;0 (zine content).

Free potassium. cyanide=Total potassium cyanide-4.0

(zinc content) Operating with such positive. free cyanide baths, it was found that bothv high-zinc. content, of'the order of. and high cathode current efiiciencies, likewiseof the. order of. 9.0%, could not be maintained simultaneously. Further, soluble alloy anodes were found to dissolve poorly in such conventional baths. Since, as. a practical matter. it isusually more expensive to replace the metal.

' content of a plating, solution with a salt than with the metal itself, a commercially successful plating process usually. requires soluble anodes.

Broadly;.in accordance. with. the present invention, it has been found'that these disadvantages may be overcome, as well as other objects realized by platingwith zinc-tinbaths having anegative-free cyanide content, that is -less than zerofree cyanide. For example,.if a bath made of'sodium salts contains less cyanide than three times the zinc content, the amount of free cyanide becomes negative. This definition of negative free cyanide. is used in the present application. The conventional definition of free cyanide has no necessary bearing to the actual. content of free cyanide present in the bath, since the zinc may be. partially complexed withhydroxide. Thus, a negative free cyanide bath may actually have some free cyanide present. It has been foundthat by using negative free cyanide baths, it is possible to deposit alloys high in zinc content at high cathode current densities. Further, Zinc-tin alloy anodes dissolve readily in. such baths.

With certain limitationsto be detailed' hereinafter, the concept of the negative free cyanide baths accordingto the present invention may be usedto deposit a solderablezinc alloy which is relatively high in zinc concentration and-contains-sufiicient tin alloy material to permitready' solderability. The solderable zinc alloy has a white matte finish which although-susceptible to finger staining exhibits the primary requisites of corrosion resistance and solderability at low cost.

The above brief description, as well as further objects, features and advantages of'the present invention .willibe; best appreciated by reference to the following detailed: description of presently preferred formulations and methods, when taken in conjunction with the accompanying drawings wherein:

Fig. 1 is a plot of the percentage of zinc in thesolderable zinc-tin alloy of the present invention for various values of sodium hydroxide and sodium cyanide in the plating bath, with a zinc concentration of 2.5 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot;

Fig. 2 isa plot of cathode current efiiciency [for variousval-ues of'sodium hydroxide and sodium cyanide with the same zinc and tin concentrations and current density as-for Fig. 1;

Fig. 3 is a plot of the percentages of zinc in the solderable zinc-tin alloy for-various values of sodium hydroxide and. sodium cyanide in theplating-zbatll'zwithha.

zinc concentration of 5 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot;

Fig. 4 is a plot of the cathode current of efiiciency for various values of sodium hydroxide and sodium cyanide in the plating bath with the same Zinc and tin concentration and current density as for Fig. 3;

Fig. 5 is a plot of the percentages of zinc in the solderable zinc-tin alloy for various values of sodium hydroxide and sodium cyanide in the plating bath with a Zinc concentration of 8 grams per liter, a tin concentration of 30 grams per liter and current density of 20 amperes per square foot;

Fig. 6 is a plot of the cathode current efficiency for various values of sodium hydroxide and sodium cyanide in the plating bath with the zinc concentration, tin concentration and current density the same as for Fig. 5;

Fig. 7 is a plot of the percentage of zinc in the solderable zinc-tin alloy for various values of sodium hydroxide and sodium cyanide in the plating bath with a zinc concentration of 12 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot;

Fig. 8 is a plot of the cathode current efficiency for various values of sodium hydroxide and sodium cyanide in the plating bath with the zinc concentration, tin concentration and current density the same as for Fig. 7;

Fig. 9 is a plot of the percentages of zinc in the solderable zinc-tin alloy for various values of sodium hydroxide and sodium cyanide in the plating bath with a zinc concentration of 16 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot; and

Fig. 10 is a plot of the cathode current efliciency for various values of sodium hydroxide and sodium cyanide in the plating bath with the zinc and tin concentrations and current density the same as for Fig. 9.

In the mentioned copending application, there are disclosed baths which were found previously to be stable chemically and permitted continued plating operations at high cathode current eificiencies (better than 85%) and high zinc alloy content (better than 87% zinc). The following formulations were listed:

Example I Zn(CN) 22.5 g./l.

Na SnO 3H O NaCN 12 g./l.

NaOH 32 g./l.

Temperature 65 C.

Cathode current density amp/sq. ft. (optimum) 540 amp./ sq. ft. (useful range) Example 2 ZnO 15.5 g./l.

Na SnO -3H O 67 g./l.

NaCN 30 g./l.

NaOH 20 g./l.

Temperature 65 C.

Cathode current density 20 amp/sq. ft. (optimum) 5-40 amp/sq. ft. (useful range) These baths, based upon sodium formulations, produced alloy platings whose composition did not change over extensive use of the bath, and which compositions were deposited at high cathode current efficiencies.

It was further disclosed that baths based upon a potassium formulation were capable of successful operation at higher current densities. Examples of the formulations of such potassium baths are as follows:

Example 3 using zinc cyanide Zn(CN) 27 g./l. K SnO -3H O 100 g./l. KOH 60 g./l. KCN 17.5 g./l. Temperature 65 C.

Cathode current density 50 amp/sq. ft. (optimum) 20-100 amp./sq. ft. (use- Cathode current density 50 amp/sq. ft. (optimum) 20-100 amp/sq. ft. (useful range) The potassium baths were disclosed as being stable and capable of sustaining operation at high cathode current efficiency of the order of at higher current densities than in the sodium baths, while producing alloy platings having a zinc content in excess of 87%.

The criteria established for the baths of the copending application, was that the resulting baths have a negative free cyanide content. This was achieved with sodium formulations by keeping the sodium cyanide content in the bath between zero and three times the Zinc metal content; and with potassium formulations by keeping the potassium cyanide content between zero and four times the zinc metal content of the bath. In both the sodium and potassium formulations, it was necessary that the hydroxide content of the bath should be at least great enough to assure that all of the zinc is complexed. The above requirements were in weight terms as follows: In the sodium bath formulation, the total sodium hydroxide content in grams per liter should not be less than 2.44 times the quantity defined as the Zinc content in grams per liter minus one-third the total sodium cyanide content in grams per liter. The maximum amount recommended was 150 grams per liter of sodium hydroxide.

When the bath formulation was based upon the use of potassium salts the total recommended amount of potassium hydroxide in grams per liter should be no less than 3.43 times the quantity defined as the zinc content in grams per liter, minus one-fourth the total of sodium cyanide content in grams per liter. The maximum value of potassium hydroxide recommended was also 150 grams per liter.

As a result of further work, it was found that the concept of negative free cyanide was insufficient to completely define the areas for the deposit of zinc-tin alloys.

At levels of sodium hydroxide of the order of 10 to 30 grams per liter, it was reconfirmed that high zinc content alloy deposits could be obtained based upon the broad concepts of said copending application. However, at higher concentrations of sodium hydroxide, specifically in the range of 75 to 150 grams per liter, the Zinc content of the alloy deposit was found to always be high and essentially independent of the cyanide concentration. Further, at very low levels of total sodium cyanide, specifically in the range of zero to 10 grams per liter, in regions of sodium hydroxide concentrations of 5 to 30 grams per liter, the zinc content of the deposit became quite small, despite the fact that the bath was a formulation based upon the negative free cyanide theory as defined in said copending application.

Several criteria have been established for preferred bath formulations in accordance with the present invention. Primary among these requirements is that the zinc content of the alloy deposit should be no less than 75% and no greater than Below 75 zinc in the zinctin alloy, the desired anodic corrosion protection dc.- creases markedly and the alloy deposit takes on the general appearance of a .tin deposit. Further, the metal cost of deposits having in excess of 25% tin is proportionately increased for higher tin contents. On the other hand, above 95% zinc in the. zinc-tin alloy, the solderability is impaired. A further and equally important. requirement is that the cathode current efficiencies be as high as possible with the ultimum of 100% to be closely approached. From a commercial standpoint, cathode current, efiiciencies over 60% are considered acceptable, if such efficiencies prevail at current densities of upto 25 amperes per square foot. for sodium formulation baths and up to 50 amperes per square foot for potassium formulation baths. Finally, to avoid the expense of replenishing the metal content of the plating bath with a salt. in lieu of the metal itself, it is important that for the desired range of zinc content and cathode current elficiency, the anodes should readily dissolve and replenish the metal ion content of the bath as needed.

In Figs. 1 to 10 inclusive, there are shown plots of the percentage of zinc in the alloy (see Figs. 1, 3, 5, 7 and 9) and the cathode current efiiciency (see Figs. 2, 4, 6, 8 and 10) for sodium formulation baths at a fixed concentration of tin of 30' grams per liter, a fixed current density of 20 amperes per square foot and various values of zinc concentration between 2.5 grams per liter to 16 grams per liter. Although these graphs are based upon sodium formulations, their interpolation to baths of potassium salts should be readily understood by those skilled in the art. That is, when using potassium salts in lieu of sodium salts, the amounts required are increased to an extent proportional to the ratio of their molecular weights. Baths formulated with sodium salts are inexpensive to install and maintain, and such sodium salts are of adequate quality and readily available. The principal disadvantage of baths based upon sodium formulations is. that of low plating speed and the rapid fall-off of current efficiency at densities exceeding 20 to 25 amperes per square foot. In that the present process may find application in the replacement of zinc, cadmium or tinzinc. plating processes, plating speeds should be comparable to known processes, in order to minimize changes in layout and time cycles. Accordingly, it is equally within the contemplation of the present invention to cover baths formulated according to the instant disclosure using salts of potassium for achieving the desired concentration of cyanide and hydroxide ion.

Figs. 1, 3, 5, 7 and 9 illustrate the desired range of zinc content in the deposit, namely 75-95%. Figs. 2, 4, 6,. 8 and 10 illustrate the desired range of cathode current efficiency, namely, 60-100%. Referring now specifically to Figs. 1 and 2, there are shown respectively plots of Zinc content of the deposit and cathode current efficiency for various values of hydroxide and cyanide content for a zinc concentration of 2.5 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot. In Fig. 1, the region enclosed bythe heavy black lines AB, BC, CD, and HA having the coordinates A, C, C and D include formulations for depositing a solderable alloy containing between 75 and 95% zinc. As is well understood by conventional coordinate notation, increasing values of sodium cyanide are measured along the abscissa or X-axis from the zero reference, increasing values of sodium hydroxide are measured along the ordinate or Y-axis fromthe zero refenence, andthe percentage of zinc in the alloy is measured along the Z-axis extending at right angles to the plane of the two coordinate system.

The approximate coordinates of the points defining the prescribed zinc content range in Fig. 1 are as follows:

A- grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, and 95 zinc in the alloy.

B0 grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, and 85% zinc in the alloy.

C5O grams per liter sodium cyanide, grams per liter sodium hydroxide and 751% zinc in alloy.

D0' grams per liter sodium cyanide, 61' grams per liter sodium. hydroxide and 75% zinc in alloy.

Although the solid line of'Fig. 1' encloses a preferred region, it is to be expressly understood that no sharp transition occurs in the zinc content in the alloy as. the outerlimits of the preferred regions are. approached. Ratherthe limits define a threshold" region wherein. the zinc content is other than within the optimum range.

In Fig. 2 there is. shown a graph of cathode current efiiciency plotted in the third dimension for varying values of sodium cyanide and sodium hydroxide shown along the abscissa and ordinate of the two coordinatesystems. In the region enclosed by the solid heavy line having the coordinates AA, BB, CC and DD, the cathode current efficiency is in excess of 60%.

The approximate coordinates of the points defining the desired region of high current efliciency are. as follows:

AA5' grams per liter of sodium cyanide,. grams per liter of sodium hydroxide, and'a. current efliciency of 60%.

BB6 grams per liter of sodium cyanide, 5 5 grams per liter of sodium hydroxide and a current efficiency of 60%.

CC--O grams per liter of sodium cyanide, 51 grams per liter of sodium hydroxide and a current efficiency of 60%.

DD-O% of sodium cyanide, 93 grams per liter of sodium hydroxide and a current efficiency of approximately 60%.

It will be appreciated by a consideration of Fig. 2'

that the cathode current eificiency for baths containing. approximately 2.5 grams per liter of Zinc is at the lower limit of the prescribed range.

By conjoint reference to Figs. 1 and 2, it is seen that there is a small overlap or common region. Within this common region, it is possible to deposit a zinc-tin alloy containing between 75' and 95% Zinc at a current efliciency in excess of 60%. However, in Figs. 1 and 2, the common area is so small and critical that it is difficult to operate a commercial plating bath within this. common region. Further, it is exceptionally difiicult to accurately control the formulation of this bath, since any drop in zinc concentration is appreciable as compared to the stated concentration of 2.5 grams per liter. As will be appreciated by progressively inspecting Figs. 3 and 4, Figs. 5' and 6, Figs. 7 and 8, and Figs. 9 and 10, it will be understood that Figs. 1 and 2 represent a formulation with zinc concentration slightly below the lowermost limit for operation in accordance with the present invention.

Referring now specifically to Figs. 3 and 4, there are shown respectively plots of zinc content in the plating and cathode current efficiency for various values of hydroxide and cyanide content for a zinc concentration of. 5 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square. foot. In Fig. 3, the region enclosed by the heavy black lines EF, FG, GI-I, HE, and havingthe coordinates, E, F, G, and H includes formulations for depositing a solderable alloy containing between 75 and 95% zinc.

The approximate coordinates of the points defining the prescribed Zinc content range of Fig. 3 are as follows:

E-Zero grams per liter sodium cyanide, grams per liter of sodium hydroxide and 82% Zinc in the alloy.

F-50 grams per liter sodium cyanide, 150 grams per liter sodium hydroxide and 95% zinc in the alloy.

G-40 grams per liter sodium cyanide, 71 grams per liter sodium hydroxide and 75 zinc in the alloy.

H-Zero grams per liter sodium cyanide, 61 grams per liter sodium hydroxide and 75 Zinc in the alloy.

Fig. 4 is a plot or graph of cathode current efliciency, drawn in three coordinates, for varying values of sodium cyanide and sodium hydroxide shown along the abscissa and ordinate of the two coordinate system. In the region enclosed by the solid heavy line having the coordinates EE, FF, GG, HH, the cathode current efliciency is in excess of 60%.

The approximate coordinates of the points defining the desired region of high current efficiency in Fig. 4 are as follows:

EEZero grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, and a current efficiency of 75%.

FF13 grams per liter of sodium cyanide, 150 grams per liter sodium hydroxide, and 60% current efiiciency.

GGl grams per liter sodium cyanide, 40 grams per liter sodium hydroxide, and 60% current efficiency.

HH-Zero grams per liter of sodium cyanide, 59 grams per liter sodium hydroxide, and a current eiiiciency of 60%.

By conjoint reference to Figs. 3 and 4, it is seen that there is a fairly extensive overlap or common region. In Fig. 3 this common region is shaded and enclosed by the solid line E1, the dot-dash line I], the solid line JH, and the solid line HE. Within this overlapping region, all formulations are capable of depositing an alloy having between 75 to 95% zinc with a current efficiency in excess of 60%. It is to be noted that the cathode current efficiency is not exceptionally high through all of the region, except for the baths having a large hydroxide content.

The approximate coordinates of the points at opposite ends of the dot-dash line I] in Fig. 3 are as follows:

I15 grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, and 91% zinc in the alloy.

J15 grams per liter sodium cyanide, 62 grams per liter sodium hydroxide and 75% Zinc in the alloy.

In Figs. 5 and 6 there are shown respectively plots of zinc content in the plating and cathode current efficiency for various values of hydroxide and cyanide content for a zinc concentration of 8 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot. In Fig. 5, the region enclosed by the heavy solid lines KL, LM, MN, NK and having the coordinates K, L, M, N include formulations for depositing a solderable alloy containing between 75 and 95% zinc. By reference to Figs. 1 and 3 it will be observed that this region is appreciably larger than the regions for lower zinc concentrations.

The approximate coordinates of the points defining the prescribed zine content range in Fig. 5 are as follows:

KZero grams per liter sodium cyanide, 140 grams per liter sodium hydroxide, 95 zinc in the alloy.

L45 grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, 95 zinc in the alloy.

M-SO grams per liter sodium cyanide, 66 grams per liter sodium hydroxide, 75 Zinc in the alloy.

N--Zero grams per liter sodium cyanide, 56 grams per liter sodium hydroxide, 75 zinc in the alloy.

O26 grams per liter sodium cyanide, 116 grams per liter sodium hydroxide and 92% zinc in the alloy.

P-24 grams per liter sodium cyanide, 25 grams per liter sodium hydroxide and 79% zinc in the alloy.

In Fig. 6 there is shown a graph of cathode current efliciency plotted in three coordinates for varying values of sodium cyanide and sodium hydroxide shown along the abscissa and ordinate of the two coordinate system. In the region enclosed by the solid heavy lines having the coordinates KK, LL, MM, NN, the cathode current efliciency is in excess of 60%.

The approximate coordinates of the points defining the desired region of high current efficiency in Fig. 6 are as follows:

KKZero grams per liter sodium cyanide, 150 grams per liter sodium hydroxide, approximately 100% current elficiency.

LIP-50 grams per liter sodium cyanide, 150 grams 8 per liter sodium hydroxide, and a current efliciency of 88%.

MM31 grams per liter sodium cyanide, 13 grams per liter sodium hydroxide and a current efiiciency of 60%.

NNZero gramsper liter sodium cyanide, 60 grams per liter of sodium hydroxide and a current efiiciency of 60%.

By conjoint reference to Figs. 5 and 6, it is seen that there is a rather substantial overlap or common region enclosed within the heavy solid lines and the dot-dash lines. Within the common area or region which is shaded, a commercial plating bath may be operated which is capable of depositing a zinc-tin alloy having a zinc concentration of between and at a current efiiciency in excess of 60%.

Referring specifically now to Figs. 7 and 8, there are shown respectively plots of the zinc content in the plating and the cathode current efficiency for various values of hydroxide and cyanide content for a zinc concentration of 12 grams per liter, a tin concentration of 30 grams per liter and a current density of 20 amperes per square foot. As before, the sodium cyanide concentration has been studied between zero and 50 grams per liter, and the sodium hydroxide concentration has been studied between approximately Zero and 150 grams per liter. In Fig. 7 the region enclosed by heavy solid lines QR, RS, ST, TQ and having the coordinates Q, R, S, T include formulations for depositing solderable Zinc alloy containing between 75 and 95 zinc; as compared to the high zinc concentration regions of Figs. 1, 3 and 5 it will be observed that the region of Fig. 7 is still more substantial.

The approximate coordinates of the points defining the prescribed zinc content range in Fig. 7 are as follows:

QZero grams per liter sodium cyanide, 134 grams per liter, sodium hydroxide, 95 zinc in the alloy.

R5() grams per liter sodium cyanide, 128 grams per liter sodium hydroxide, and 95% in alloy.

S5O grams per liter sodium cyanide, 66 grams per liter sodium hydroxide, and 75% zinc in the alloy.

T-Zero grams per liter sodium cyanide, 55 grams per liter sodium hydroxide and 75% zinc in the alloy.

In Fig. 8 there is shown a graph of cathode current efiiciency plotted in three coordinates for varying values of sodium cyanide and sodium hydroxide for the formulations of Fig. 7. In the region enclosed by the solid heavy lines having the coordinates QQ, RR, SS, 'IT, the cathode current efiiciency is in excess of 60%.

The approximate coordinates of the points defining the desired high current efficiency in Fig. 8 are as follows:

QQ-Zero grams per liter sodium cyanide, 150 grams per liter sodium hydroxide and current efiiciency of approximately RR-50 grams per liter sodium cyanide, grams per liter sodium hydroxide and a current efficiency of approximately 98%.

SS5O grams per liter sodium cyanide, 84 grams per liter sodium hydroxide and a current efliciency of 60%.

TT-Zero grams per liter sodium cyanide, 64 grams per liter sodium hydroxide and a current efficiency of 60%.

By conjoint inspection of Figs. 7 and 8, it will be seen that the entire high zinc content region of Fig. 7 has overlapping portions in the high current efiicicncy region of Fig. 8 and accordingly all formulations suitable for depositing high zinc shown in Fig. 7 have current efliciencies in excess of 60%. It will be appreciated that the range of formulations is exceptionally broad and that it is thus possible to operate a commercial plating bath Without extreme control over variations in the bath formulation.

I Referring now to Figs. 9 and 10, there are shown respectively graphs of zinc content in plating and cathode curitent efiiciency forvarious values of hydroxide and cyanide content for a zincconcentration of 16 grams per liter, a tin concentration of 30* grams per liter, and a current. density of ZOwamperes per square foot. In- Fig. 9,; the region enclosed by the heavy black lines UV, VW, WX, and" XU' having the coordinates U, V, W, X include formulations for depositing a, solder-able alloy containing between, 75' and 95% zinc. As compared to the. showings of" Figs. 1, 3 and 5-, it will be appreciated that the region of Fig. 9' is somewhat more extensive, and comparable to the region shown in Fig, 7.

The approximate coordinates of the points defining the prescribed high zinc content range in Fig. 9 are as follows:

U.'Zero grams per'liter of sodium cyanide, 105 grams per, liter sodium hydroxide and 95% zinc in the alloy.

V--50 grams per liter sodium cyanide, 148 grams per liter sodium hydroxide, and 9.5% zinc in the. alloy.

W-50 grams per liter sodium cyanide, 40 grams per liter sodium hydroxide and 75% zinc in. the alloy.

XZero. grams per liter sodium. cyanide, 47 grams per liter sodium hydroxide and 75% zinc in the alloy.

In. Fig. 1.0 there is shown -a plot of cathode current efficiency for varying values of sodium cyanide and sodiunr hydroxide for the parameters established in conjunction with: Fig. 9. In the region enclosed by the solid heavy lines having the coordinates UU, VV, WW, and XX the cathode current efliciency is in excess of 60%.

The approximate coordinates of the points defining the desired region of high current efiiciency in Fig. 10 are as follows:

UUZero grams per liter sodium cyanide, 150 grams per liter sodium hydroxide and a current efiiciency of approximately 100%.

VV-50 grams per liter sodium cyanide, 150 grams per liter sodiumhydroxide, and a current efiiciency of approximately 95%.

WW50 grams per liter sodium cyanide, 33 grams per liter" sodiumhydroxideand a cur-rent. efficiency of 60%.

XXZero grams per liter sodiumcyanide, 70 grams per liter sodium hydroxide and a current efiiciency of 60%.

By conjoint reference to Figs. 9 and 10 it will be seen that the high plating region of Fig. 9 is completely contained within the region of high current eficiencies and accordingly all solutions contained within. the shaded region. UV, VW, WX, and XY of Fig. 9 are acceptable for producing a zinc tin alloy containing between 75 and 95% zinc at, a current efliciency in excess of 60%.

It is to bestressed that the graphs of Figs. 1 to 10 inclusive are merely intended for illustrative purposes anddo not completely cover the operative regions according to the; present disclosure. Further, the graphs merely illustrative the sodium formulation baths and are for a fixed concentration of tin, and a fixed current density. The broader concepts of the present invention involve the use of either a sodium or potassium formulation bath which will deposit an alloy containing between 75 and 95% zinc balance tin, at cathode efiiciencies of 60 to 100%, and at current densities of up to 25' amperes per square foot for sodium baths and 50 amperes for potassium baths, with the zinc concentration varying between the limits of 5 and 50 grams per liter and the tin concentration varying between the limits of to 75 grams per liter. For all formulations, the total amount of hydroxide and cyanide content present should be sufficient to complex all of the zinc as a complex zinc radical.

The optimum temperature of operation is 65 C. with a range of from 45 to 85 C. At temperatures lower than 60 C., the current efiiciency falls off sharply. Increasing the temperature will not adversely affect the efl'iciency, but will increase the rate of decomposition of salts-and the. heating costs.

For all formulations in accordance with the present invention, it is possible to employ soluble anodes to; re plenish the metal content of the plating solution. The optimum composition of the anodes is by weight zinc and 25% by weight tin; anode current density and current efficiency are such that all of the tin andpart of the; zinc plated out of" the solution is replenished. The balance of the zinc may customarily be replaced by adding zinc oxide to the solution. The treatment of the anodes is detailed in copending application Serial No. 426,046, filed April 27, 1 954 and assigned to the assignee of the present invention, and involves placing the anodesin' the plating tank and increasing the tank current until all the anodes are polarized. The tank current is then reduced to its proper value for plating. For the formulations previously disclosed, anodes may be operated at-high efficiencies in excess of at ten amperes per square foot for sodium baths and at 25 amperes per square foot for potassium baths. The polarizing current density for newly cast anodes varies. In an unagitated bath, thepolarizing'currentdensity is about 65 amperes per square foot for sodium baths and 1-00 amperes per square foot for potassium baths at the optimum temperature of 65 C.

It has been found that the cyanide content and hydroxide content are somewhat interdependent. From zero to five grams per liter sodium cyanide, the hydroxide content should be greater than 60 grams per liter sodium hydroxide; from 5 to 10 grams per liter sodium cyandie the hydroxide content should be greater than 40 grams per liter sodium hydroxide; and for 10 grams per liter sodium cyanide to three times the zinc concentration, the hydroxide content should be greater than 25 grams per liter sodium hydroxide. Similarly, for potassium formulations between zero and 6.5 grams per liter potassium cyanide, the hydroxide content should be greater than 75 grams per liter potassium hydroxide; from 6.5 grams to 13 grams per liter sodium cyanide, the hydroxide content; should be greater than 55 grams per liter potassium hydroxide; and from 10 grams per liter potassium cyanide to four times the zinc concentration, the hydroxide content should be greater than 35 grams per liter potassium hydroxide. At all times the total amount of hydroxide and cyanide present should be sufficient to fully dissolve the zinc by complexing action.

It has been further found that for zinc contents in excess of 8 grams per liter, there is an upper limit to the hydroxide content; Specifically for 8 grams per liter zinc, the upper limit is approximately grams per liter sodium hydroxide (or, 175 grams per liter of potassium hydroxide); for 12 grams per liter of zinc, the upper limitis about 100 grams per liter of sodium hydroxide (or, grams per liter of potassium hydroxide); and for 16 grams per liter of zinc, the upper limit is about 75 grams per liter sodium hydroxide (or, 105 grams per liter of potassium hydroxide). All of the above formulations are based upon the sodium cyanide content carrying between.

zero and 25 grams per liter. With increasing cyanide concentration the upper limit for the hydroxide content increases to approximately grams per liter at Zero free cyanide. By interpolating the above results, appropriate limits may be found for baths based upon potassium formulations;

The solderable zinc alloy produced in accordance, with the present baths and methods has a white matte finish which is rather prone to finger staining. In salt-fog atmosphere the alloy is also prone to white salt formation although to a slightly lesser extent that pure zinc. For these reasons it may be desirable to coat the alloy with appropriate finishing dips. A conventional dip for this purpose consists of immersing the alloy in 1% by weight of nitric acid, which produces a very thin oxide film of a pleasing gray color. This film suppresses finger staining While not interfering with solderability and any cyanide left after plating and rinsing is neutralized by this con- 75 ventionaltdip. Similarly chromate conversion coatings may be applied to the deposits, but unlike the nitric acid dips, the chromate film increasingly interferes with solderabilityas its thickness is increased.

Numerous modifications and substitutions in the present process and baths will occur to those skilled in the art. Accordingly, the appended claim should be given a latitude of interpretation consistent with the disclosure; at times certain features of the invention will be used without a corresponding use of other features.

What we claim is:

1. A cyanide-type of plating bath for depositing a zinc-tin alloy comprising in solution zinc, tin, a cyanide ion source and a hydroxide ion source, the cyanide ion being present in an amount less than that required to complex all of the zinc as cyanozincate ion, the total amount of cyanide ion and hydroxide ion being sufiicient to complex all of the zinc.

2. A cyanide-type of plating bath for depositing a zinc-tin alloy comprising in solution zinc, tin, sodium cyanide and sodium hydroxide, the cyanide ion being present in an amount less than that required to complex all of the zinc as cyanozincate ion, the total amount of cyanide ion and hydroxide ion being sufiicient to complex all of the zinc.

3. A cyanide-type of plating bath for depositing a zinc-tin alloy comprising in solution zinc, tin, potassium cyanide and potassium hydroxide, the cyanide ion being present in an amount less than that required to complex all of the zinc as cyanozincate ion, the total amount of cyanide ion and hydroxide ion being sufiicient to complex all of the zinc.

4. A plating solution for electrolytically forming a zinc-tin alloy containing about 87 percent zinc comprising an alkaline cyanide, an alkaline hydroxide, zinc and tin, the cyanide ion being present in a concentration less than required to complex all of the zinc as Zn(CN) the total concentration of cyanide and hydroxide ion being sufiicient to complex all of the zinc, the Zinc concentration being between 5 and 50 grams per liter, and the tin concentration being between 15 and 75 grams per liter.

5. A plating solution for electrolytically forming a zinc-tin alloy containing about 87 percent zinc comprising sodium cyanide, sodium hydroxide, zinc and tin, the cyanide ion being present in a concentration less than required to complex all of the zinc as Zn(CN) the total concentration of cyanide and hydroxide ion being sufiicient to complex all of the zinc, the zinc concentration being between 5 and 50 grams per liter and the tin concentration being between 15 and 75 grams per liter.

6. A plating solution for electrolytically forming a zinc-tin alloy containing about 87 percent zinc comprising potassium cyanide, potassium hydroxide, zinc and tin, the cyanide ion being present in a concentration less than required to complex all of the Zinc as Zn(CN) the total concentration of cyanide and hydroxide ion being sufiicient to complex all of the zinc, the zinc concentration being between 5 and 50 grams per liter and the tin concentration being between 15 and 75 grams per liter.

7. A process for the electrolytic deposition of a solderable zinc-tin alloy containing in excess of 87 percent zinc including the steps of subjecting an article to be plated, as cathode, to electrolysis in a plating bath containing 5 to 50 grams per liter of zinc, 15 to 75 grams per liter of tin and both cyanide and hydroxide ion, the cyanide ion being present in an amount insufiicient to complex all of the zinc as cyanozincate ion, the total cyanide and hydroxide ion concentration being suflicient to complex all of the zinc, while maintaining an average current density of 20 to 40 amperes per square foot and a plating bath temperature of approximately 65 C.

8. A process for the electrolytic deposition of a solderable zinc-tin alloy containing about 87 percent zinc including the steps of subjecting an article to be plated, as

cathode, to electrolysis in a plating bath containing 5 to 50 grams per liter of zinc, 15 to grams per liter of tin and sodium cyanide and sodium hydroxide, the cyanide ion being present in an amount insuflicient to complex all of the zinc as cyanozincate ion, the total cyanide and hydroxide ion concentration being sufficient to complex all of the zinc, while maintaining an average current density of 5 to 40 amperes per square foot and a plating bath temperature of approximately 65 C.

9. A process for the electrolytic deposition of a solderable zinc-tin alloy containing about 87 percent zinc including the steps of subjecting an article to be plated, as cathode, to electrolysis in a plating bath containing 5 to 50 grams per liter of zinc, 15 to 75 grams per liter of tin and potassium cyanide and potassium hydroxide, the cyanide ion being present in an amount insufiicient to complex all of the zinc as cyanozincate ion, the total cyanide and hydroxide ion concentration being suflicient to complex all of the zinc, while maintaining an average current density of 20 to 100 amperes per square foot and a plating bath temperature of approximately 65 C.

10. A zinc-tin alloy plating bath made up of zinc, tin, and sodium salts comprising sodium hydroxide and sodium cyanide, in which the total sodium hydroxide content in grams per liter is not less than 2.44 times the zinc content of the bath in grams per liter minus /3 the total sodium cyanide present in grams per liter.

11. A zinc-tin alloy plating bath made up of zinc, tin, and potassium salts comprising potassium hydroxide and potassium cyanide, and which contains potassium hydroxide in grams per liter not less than 3.43 times the zinc content of the bath in grams per liter minus the total potassium cyanide content in grams per liter.

12. An alloy plating bath made up of zinc, tin, a source of hydroxide ions and a cyanide, whose total cyanide content is less than the amount required to complex all of the zinc as Zn(CN) the zinc content lying between 5 and 50 grams per liter and the tin content lying between 15 and 75 grams per liter.

13. A zinc-tin alloy plating bath made up of zinc, tin, and sodium salts comprising sodium hydroxide and sodium cyanide, in which the sodium cyanide content of the bath lies above zero and below three times the zinc metal content and in which the zinc content lies between 5 and 50 grams per liter and the tin content lies between 15 and 75 grams per liter,

14. A zinc-tin alloy plating bath made up of zinc, tin, and potassium salts comprising potassium hydroxide and potassium cyanide, in which the potassium cyanide content of the bath lies above zero and below three times the zinc metal content and in which the zinc content lies between 5 and 50 grams per liter and the tin content lies between 15 and 75 grams per liter.

15. A zinc-tin alloy plating bath made up of zinc, tin, and sodium salts comprising sodium hydroxide and sodium cyanide, and in which the total sodium hydroxide content in grams per liter is not less than 2.44 times the zinc content of the bath in grams per liter minus 6 the total sodium cyanide present in grams per liter, the zinc content lying between 5 and 50 grams per liter and the tin content lying between 15 and 75 grams per liter.

16. A zinc-tin alloy plating bath made up of zinc, tin, and potassium salts comprising potassium hydroxide and potassium cyanide, and which contains potassium hydroxide in grams per liter not less than 3.43 times the zinc content of the bath in grams per liter minus the total potassium cyanide content in grams per liter, the zinc content lying between 5 and 50 grams per liter and the tin content lying between 15 and 75 grams per liter.

17. A plating solution for electrolytically forming a zinc-tin alloy containing between 75 and percent zinc comprising an alkaline cyanide, an alkaline hydroxide, zinc and tin, the cyanide ion being present in a concentration less than that required to complex all of the zinc as cyanozincate ion and in a concentration greater than,

five grams per liter, the total concentration of cyanide and hydroxide ion being suflicient to complex all of the zinc, the zinc concentration being between the limits of to 50 grams per liter and the tin concentration being between the limits of 15 to 75 grams per liter.

18. A plating solution for electrolytically forming a zinc tin alloy containing between 75 and 95 percent zinc com prising sodium cyanide, sodium hydroxide, zinc and tin, the cyanide ion being present in a concentration less than that required to complex all of the zinc as cyanozincate ion and in a concentration greater than ten grams per liter, the total concentration of cyanide and hydroxide ion being suificient to complex all of the zinc, the zinc concentration being between the limits of 5 to 50 grams per liter and the tin concentration being between the limits of 15 to 75 grams per liter.

19. A plating solution for electrolytically forming a zinc-tin alloy containing between 75 and 95 percent zinc comprising potassium cyanide, potassium hydroxide, zinc and tin, the cyanide ion being present in a concentration less than that required to complex all of the zinc as cyanozincate ion and in a concentration greater than thirteen grams per liter, the total concentration of cyanide and hydroxide ion being sufficient to complex all of the zinc, the zinc concentration being between the limits of 5 to 50 grams per liter and the tin concentration being between the limits of 15 to 75 grams per liter.

20. A process for the electrolytic deposition of a solderable zinc-tin alloy containing from 75 to 95 percent zinc and 5 to 25 percent tin including the steps of subjecting an article to be plated, as cathode, to electrolysis in a plating bath containing 5 to 50 grams per liter of zinc, 15 to 75 grams per liter of tin, and cyanide and hydroxide ion, the cyanide ion being present in an amount less than that required to complex all of the zinc as cyanozincate ion and in an amount greater than five grams per liter, the total cyanide and hydroxide ion concentration being sufficient to complex all of the zinc, while maintaining a current density of between five and one hundred amperes per square foot. and a plating bath temperature of between 45 C. and 85 C.

21. A process for the electrolytic deposition of a solder able zinc-tin alloy containing from 75 to 95 percent zinc and 25 to 5 percent tin including the steps of subjecting an article to be plated, as cathode, to electrolysis in a plating bath containing 5 to 50 grams per liter of zinc, 15 to 75 grams per liter of tin, sodium cyanide and sodium hydroxide, the sodium cyanide being present in an amount less than that required to complex all of the zinc as cyanozincate ion and in an amount greater than ten grams per liter, the total cyanide and hydroxide concentration being sufiicient to complex all of the zinc, while maintaining a current density of five to forty amperes per square foot, and a plating bath temperature of between C. and 85 C.

22. A process for the electrolytic deposition of a solderable zinc-tin alloy containing from 75 to 95 percent zinc and 25 to 5 percent tin including the steps of subjecting an article to be plated, as cathode, to electrolysis in a plating bath containing 5 to grams per liter of zinc, 15 to grams per liter of tin, potassium cyanide and potassium hydroxide, the potassium cyanide being present in an amount less than that required to complex all of the zinc as cyanozincate ion and in an amount greater than thirteen grams per liter, the total cyanide and hydroxide concentration being sufiicient to complex all of the zinc, while maintaining a current density of twenty to one hundred amperes per square foot, and a plating bath temperature of between 45 C. and C.

References Cited in the file of this patent UNITED STATES PATENTS 1,904,732 Haueisen et al. Apr. 18, 1933 2,600,352 Wernlund June 10, 1952 2,675,347 Lowenheim Apr. 13, 1954 FOREIGN PATENTS 511,841 Belgium Dec. 3, 1952 548,009 Great Britain Sept. 21, 1942 

1. A CYANIDE-TYPE OF PLATING BATH FOR DEPOSITING A ZINC-TIN ALLOY COMPRISING IN SOLUTION ZINC, TIN, A CYANIDE ION SOURCE AND A HYDROXIDE ION SOURCE, THE CYANIDE ION BEING PRESENT IN AN AMOUNT LESS THAN THAT REQUIRED TO COMPLEX ALL OF THE ZINC AS CYANOZINCATE ION, THE TOTAL AMOUNT OF CYANIDE ION AND HYDROXIDE ION BEING SUFFICIENT TO COMPLEX ALL OF THE ZINC. 